1. Field of the Invention
The present invention relates to an optical resonator and a tunable laser.
2. Description of the Related Art
Tunable lasers are widely used to generate light of different wavelengths in wavelength division multiplexed communication systems, because they enable a single device to handle a plurality of wavelengths. Numerous tunable laser structures have been proposed, as disclosed in, for example, Japanese Patent Application Publications No. H10-261837, 2000-261086, 2000-223744, 2005-327881, 2002-6352, 2004-71809, 2006-196554, 2007-115900, and 2007-234786, and by Takahashi et al. in ‘Tunable Lasers Based on Silica Waveguide Ring Resonators’, Optical Fiber Communication Conference Papers 2008, OWJ1.
A common feature of the tunable lasers in these disclosures is that they include a semiconductor optical amplifier and a tunable filter in an optical resonator.
Three requirements are now being imposed on tunable lasers: small size, a small number of components, and easy optical alignment. The tunable laser proposed by Takahashi et al., which employs an optical waveguide structure, is attracting attention as a way of meeting these requirements.
The tunable laser disclosed by Takahashi et al. uses two ring resonators with different free spectral ranges, and enables the wavelength to be tuned by the Vernier effect of the wavelength transmission peak. A ring resonator for supplying a reference wavelength is also included; used together with the Vernier effect, it enables the wavelength band of the selected light to be varied over a comparatively wide range of fifty nanometers (50 nm) with only a slight change in refractive index. This tunable laser is made from materials with a high refractive index, so the radius of curvature of the optical waveguide can be small enough to reduce the laser device to a square measuring about five millimeters (5 mm) on each side.
The resonant wavelengths of the two ring resonators in the tunable laser disclosed by Takahashi et al. must, however, be precisely controlled, requiring a complex control circuit. There is moreover a need for an optical resonator even smaller than the optical resonators in the tunable laser disclosed by Takahashi et al.